1. Field of the Invention
The present invention relates to a planographic printing plate precursor, and more particularly, to a planographic printing plate for laser plate production.
2. Description of the Related Art
Recently, with development of image forming technologies, attention has been focused on technologies for forming letter manuscripts, images and the like directly on the surface of a plate, while scanning the plate with laser beams restricted narrowly, to produce a plate directly without using a film.
As such an image forming material, there are listed a so-called thermal type positive type planographic printing plate in which an infrared absorbing agent present in a photosensitive layer generates heat upon exposure by exhibiting its light-heat converting action, and exposed portions of the photosensitive layer are solubilized by the generated heat to form positive images, and a thermal type negative type planographic printing plate of in which a radical generator and an acid generator generate a radical and an acid due to heat, and a radical polymerization reaction and an acid crosslinking reaction occur, causing insolubilization of exposed portions of the photosensitive layer, to form negative images. In such thermal type image formation, laser light irradiation causes a light-heat converting substance in a photosensitive layer to generate heat which causes an image formation reaction.
A planographic printing plate precursor which enables laser plate printing (direct type planographic printing plate precursor) is generally manufactured by roughening the surface of an aluminum plate which is in the form of a wave, carrying out an anodizing treatment on the surface, and then applying thereon a photosensitive layer coating solution and drying it, to form a photosensitive layer. Then, the planographic printing plate precursor in the form of a wave is cut into a sheet of desired size, and a plurality of such sheet are stacked and then packed. Alternatively, after being stored in a state of being wound in roll form, the plate is cut into desired sizes. The packed and delivered planographic printing plate precursors are subjected to image printing by laser exposure and to developing processing, and are then set at a printer.
However, an aluminum substrate which has been roughened and on which an anodized film has been formed essentially has the problem of low sensitivity for the following reason. Because the substrate has heat conductivity which is extremely high as compared with that of the photosensitive layer, heat generated in a vicinity of the interface between the photosensitive layer and the substrate is diffused into the substrate before being used for forming images sufficiently, resultantly. As a result, the decomposition reaction of the positive photosensitive layer is insufficient at the interface between the photosensitive layer and the substrate, and a film remains at the non-image parts.
Further, there is also the problem that although such a thermal type recording layer must contain an infrared absorbing agent having light-heat converting ability, such agents have poor solubility due to their relatively large molecular weight, and adhere to micro openings in the anodized substrate and are difficult to be removed therefrom. Therefore, a film tends to remain in a developing process using an alkali developing solution.
For coping with this problem, various primers have been studied, for improving the developing property of the photosensitive layer at the interface between the substrate and the photosensitive layer, in the case of a positive photosensitive layer. However, a sufficiently satisfactory level has not been attained in any case.
When roughening of a substrate is non-uniform, the tight contact between the photosensitive layer and the substrate also decreases. When the close fit between the photosensitive layer and the substrate decreases, the ability to withstand repeated printings of a planographic printing plate after plate production is lowered. Particularly, with a photosensitive layer of a direct writing type planographic printing plate, it is difficult to ensure close contact with a substrate as compared with a photosensitive layer of a planographic printing plate requiring a plate production film in the production thereof. Thus, an improvement in the ability to withstand repeated printings is desired.
Further, recently, sensitive materials which are activated by a shorter wavelength as compared with conventional products which are activated by wavelengths around 500 nm have been studied for enabling work under a safe light of a bright red color. However, in the photosensitive printing plate which is activated by a short wavelength of 450 nm or less and is described in Japanese Patent Application No. 11-209822 and has been newly developed recently, light absorption of an anodized film at an exposure wavelength of 450 nm or less is low as compared with the absorption at wavelengths around 500 nm. Therefore, in conducting laser image writing on a printing plate, the plate tends to be affected by light diffusion, and a thin image portion called a fringe is formed around each halftone dot. Consequently, a problem occurs that the halftone dot on the whole becomes bolder, and the halftone dot area ratio increases.
In this case, it is advisable to further increase the light absorption of the anodized film, and to this end, it is necessary to raise the volume proportion of the anodized film itself by decreasing the pore diameter of fine pores called micropores existing in the anodized film, or by decreasing the number of pores per unit area. However, on the other hand, since the micropores of an anodized film of aluminum result in close contact by holding the photosensitive layer by an anchor effect, a decrease in the size of the micropores or a decrease in the number of micropores per unit area thus deteriorates the close contact with the photosensitive layer, such that the structure cannot be used in actual practice. Therefore, for obtaining close contact by the substrate, the presence of a certain amount of micropores is necessary. Until now, there has been no way other than sacrificing halftone dot quality and reproducibility in order to form an image and using it as a printing plate.
In addition, in the above-described packaging of direct writing type planographic printing plate precursors, it is necessary to precisely stack the plurality of planographic printing plate precursors cut to the same given size. To this end, it is necessary to precisely convey the plurality of planographic printing plate precursors cut into the same given size. For the conveying, a belt conveyer is usually used. However, there is the problem that a planographic printing plate precursor may slip, and accurate conveying and stacking are difficult. Further, though conveying belts and conveying rollers are used for laser image writing, development, printing and the like conducted by users, and also for the transfer of the planographic printing plate precursor to various processes, there is a problem that the planographic printing plate precursor may slip and accurate conveying and stacking are difficult with these conveying belts and conveying rollers as well. Particularly in laser exposure, extremely high positioning accuracy is required, and therefore, poor conveying invites not only a reduction in productivity but also a reduction in the quality of formed images. Also, in developing processing, a automatic conveying type developing machine are used in almost all cases, and there is a great demand to overcome the problem of poor conveying during the developing process as well.
An object of the present invention is to provide a direct writing type planographic printing plate precursor which can overcome the above-described various problems.
The present inventors conducted intensive studies, and found that the above-described object can be attained by using an aluminum substrate having specific properties, and thus arrived at the present invention.
A planographic printing plate precursor of the present invention comprises: an aluminum substrate which has been subjected to a roughening treatment and an anodizing treatment; and a photosensitive layer which is provided on a surface of the substrate, and which contains an infrared absorbing agent and a water-insoluble and alkali aqueous solution-soluble polymer compound, and whose solubility in an alkali developing solution varies by infrared laser exposure; wherein the substrate is obtained by electrochemically roughening an aluminum alloy plate which contains 0.05 to 0.5% by weight of Fe, 0.03 to 0.15% by weight of Si, 60 to 300 ppm of Cu, 100 to 400 ppm of Ti and 10 to 200 ppm of Mg, contains 1 to 100 ppm of at least one element selected from the group of elements consisting of Li, Na, K, Rb, Cs, Ca, Sr, Ba, Sc, Y, Nb, Ta, Mo, W, Tc, Re, Ru, Os, Co, Rh, Ir, Pd, Pt, Ag, Au, C, Ge, P, As, S, Se, Te and Po, and has an aluminum purity of 99.0% by weight or more.
As a result of various studies, the present inventors found that by adding a trace amount of at least one of the above-listed elements to an aluminum alloy of high purity, uniform roughening can be achieved when carrying out an electrochemical roughening treatment, and thus arrived at the present invention.
In a given aspect, in order to achieve the above-described object, the planographic printing plate of the present invention comprises the above-described substrate and the above-described photosensitive layer, and the substrate has at least one of following features (a) and (b):
(a) the substrate has an average roughness Ra at the center line of 0.5 xcexcm or less, and has a surface area of 2 times to 30 times a unit surface area, (b) micropores present in an anodized film on the above-described substrate have a pore diameter of 1 to 5 nm and a pore density of 8xc3x971015 to 2xc3x9710/m2.
The aluminum substrate (a) having a surface area which is 2 times to 30 times a unit surface area can be easily obtained by a method in which a micropore sealing treatment is conducted after the anodizing treatment, or other methods. According to the present invention, by decreasing the surface roughness Ra of a roughened substrate, the thickness of the coated photosensitive layer is uniform, local formation of the thick photosensitive layer regions in which heat generation by laser light absorption does not easily occur is prevented, and sensitivity can be efficiently enhanced.
Usually, a surface area obtained by actual measurement is from 40 to 100 times the apparent surface area of a surface which is used for printing and which has been roughened by anodized film used as a substrate for a planographic printing plate. However, in the present invention, by making the relation therebetween fall in a range from 2 to 30 times and thus decreasing the surface area, the depth and size of micropores in the anodized film layer are controlled. Absorption of an infrared absorbing agent having a large molecular weight, and formation of a photosensitive layer which invades into deep parts of the micropores and is not removed easily by a developing solution can be prevented. Generation of residual film is suppressed, and the micropores in the anodized film layer work as independent heat insulation layers respectively. Consequently, heat conductivity at the interface of the photosensitive layer and the substrate decreases, and generated heat is efficiently used for an image formation reaction, thus leading to enhancement of sensitivity.
Conventionally, there is also a method used in some cases, wherein the surface area of a substrate for a printing plate is decreased by a micropore sealing treatment using a pressurized water vapor treatment or a hot water treatment for the purpose of decreasing remaining color. However, the effect obtained by the present invention cannot be obtained merely by a micropore sealing treatment. In the present invention, the excellent effect of the present invention can be attained by controlling the surface area of the substrate to fall within a range of 2 to 30 times the apparent surface area, by use of a micropore sealing treatment or another treatment method. Further, it has been found that by controlling the surface roughness (Ra) to fall in the preferable range of less than 0.5 xcexcm, local reduction in sensitivity due to non-uniform thickness of the photosensitive layer can be suppressed, and uniform high sensitivity over the entire region of the photosensitive layer can be attained.
Further, a given aspect of the planographic printing plate precursor of the present invention for attaining the above-described object is a planographic printing plate precursor comprising the substrate and the above-described photosensitive layer, wherein the reverse surface of the substrate has different average surface roughnesses Ra along the longitudinal direction and the transverse direction, and given that the average surface roughness Ra along the direction of the larger average surface roughness is represented by Ral and the average surface roughness Ra along the direction of the smaller average surface roughness is represented by Ras, Ral and Ras satisfy the following relational formula:
1.1xe2x89xa6Ral/Rasxe2x89xa65.0
In the planographic printing plate precursor of the present aspect, the reverse surface of the substrate has average surface roughnesses Ra which are mutually different along the longitudinal direction and the transverse direction, and Ral and Ras satisfy the above-described relational formula. When the planographic printing plate precursor of the present aspect is conveyed by a conveyor belt or conveyor roller, different frictional forces act along the longitudinal direction and the transverse direction on the reverse surface of the substrate. Due to the action of the frictional forces which are mutually different along the longitudinal direction and the transverse direction on the reverse surface of a substrate, slipping and meandering in conveying can be effectively prevented. (Here, xe2x80x9cmeanderingxe2x80x9d means the precursor being conveyed at an angle with respect to the direction in which it should be conveyed.)
Furthermore, a given aspect of the present invention for attaining the above-described object is a planographic printing plate precusor comprising a substrate and photosensitive layer which has laser light sensitivity and id provided on the substrate, wherein the reverse surface of the substrate is subjected to a light degree of surface at least in a region located from the end of one side of the reverse surface of the substrate and having a width of 1 mm or more and 50 mm or less.
In the planographic printing plate precursor of this aspect, the reverse surface of a substrate has at least a lightly roughened region of a predetermined width at the end of one side. When the planographic printing plate precursor of this aspect is conveyed by a conveying belt or conveying roller, frictional forces which is mutually different at the lightly roughened region and non-roughened regions act on the reverse surface of the substrate. Due to the action of the large frictional force at the end of the reverse surface of the substrate, slipping and meandering during conveying can be effectively prevented.
In the planographic printing plate precursors of the above-described two aspects, when the photosensitive layer is a photosensitive layer which is scratched in a test by using a scratch tester (sapphire needle, 0.5 mmxcfx86) using a load of 30 g, it is preferable to form an anodized film of 0.1 g/m2 or more on the reverse surface of the substrate.
When the planographic printing plate precursors of the above-described two aspects are stacked and stored, if the reverse surface comes into contact with a photosensitive layer, the photosensitive layer is not locally scratched, since the reverse surface has a certain degree of irregularity uniformly over the entire surface thereof. However, if a part of the reverse surface of one precursor is scratched, when the precursors are stacked and stored, the photosensitive layer tend to be locally scratched. The same tendency occurs also when a precursor is wound in the form of a roll and stored. Therefore, by forming an anodized film of 0.1 g/m2 or more on the reverse surface, the surface hardness of the reverse surface increases, and as a result, the reverse surface is not scratched easily. When the precursors are stacked and stored or when wound in the form of a roll and stored, scratching of the photosensitive layer can be prevented.